Nitrification of waste waters from fish-meal factories

Fish-meal factories generate waste waters with high chemical oxygen demand (COD), suspended solids (SS), fats and protein concentrations. After their treatment by anaerobic digestion, effluents with high ammonia- and organic-nitrogen contents and a fraction of residual COD are produced. A biological nitrification/denitrification process is proposed to reduce both the residual carbon and the nitrogen. The aerobic stage has the double objective of reducing COD and nitrifying the ammonia- and organic-nitrogen for its further removal at the denitrification stage. The nitrification process of the waste water is presented here. Under operating conditions, ammonia accumulation in the aerobic reactor occurs, inhibiting nitrite-oxidising bacteria. Specific removal rates of COD (up to 1.5 g COD/g VSS.d) and TKN (up to 0.25 g N/g VSS.d) were obtained. The effect of nitrogen loading rate (NLR) on ammonia conversion was studied. A linear relationship was found between NLR and ammonia oxidation rate up to NLR of 0.6 g-TKN/l.d, being the slope 0.57 g/g and the correlation factor of 0.96. With an NLR of higher than 0.6, it was not possible to obtain a nitrification rate higher than 0.5 g NO x - /l.d. Percentages of COD removal ranged from 95% to 60% and the nitrification percentages from 65% to 20%.

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